The proposed research investigates developmental changes in spoken word recognition that occur with vocabulary growth. These changes will be assessed through cross-sectional comparisons, and one mini-longitudinal study spanning a six-month period. The focus is on early and middle childhood, because relatively little is known about spoken word re cognition in this period. Four specific aims guide the proposed investigation. The first aim is to better understand changes in the structural representations of spoken words and their acoustic-phonetic similarity relations to one another (a) as the lexicon grows in overall size, (b) as particular regions become more "crowded", and (c) as individual items become more familiar. The research will show, for example, whether the phonological representations of "early"-acquired words are more "complete" than those of "later"- acquired ones. The second aim is to identify changes in the level of processing at which word familiarity and overall vocabulary growth influence recognition. The research will indicate whether familiarity has its greatest impact on the early, perceptual encoding of words by young listeners, and only later becomes higher-level knowledge that biases recognition decisions. Information about listeners' conscious awareness of the familiarity status, and the segmental structure of their lexical representations will also be obtained. The third aim is to learn how certain task characteristics (e.g., memory requirements, the presence or absence of contextually constraining information) contribute to developmental differences in spoken word recognition performance. The fourth aim is to develop a new recognition task that is more sensitive than existing ones to the receptive vocabulary knowledge of young children with limited speech production ability. These objectives will be addressed in seven experiments with native English-speaking children (aged 3 to 10) and adults. The experiments will provide important new information about the nature and basis of developmental changes in spoken word recognition, which is central to fluent speech comprehension.